The advent of scanning force microscopy (SFM), also known as atomic force microscopy (AFM), has brought an instrument capable of microscopic surface studies with atomic resolution, suited for ambient and liquid environments and a wide variety of samples. SFM is a method for observing nanoscale topography and other properties of a surface. In general, SFM scans a force sensor over a surface.
SFM can be carried out in contact and non-contact modes. In a contact mode of operation, a topographical image is produced by measuring the deflection of a small cantilever beam which has a sharp probe attached to its free end. Higher areas of the surface deflect the cantilever more. This deflection is typically detected by reflecting a laser beam off of the back of the cantilever onto a photodiode that is connected to provide its output signal to a computer, which converts the signal into a number. In a constant height mode, the height of the scanner is constant and the cantilever deflection can be used directly to generate the topographical data. In a constant force mode, the height of the probe above the surface is adjusted until the cantilever deflection value reaches a setpoint. The image is generated from the scanner height data. As the cantilever probe scans the surface, an image is produced based on the height of the scanner, pixel by pixel, with the darkness of each pixel representing the height data at that pixel.
Non-contact modes differ from the contact mode in that the cantilever is driven to oscillate, typically at its resonant frequency, and the amplitude, phase or frequency or a combination of these parameters is measured, e.g., by a laser beam and photodiode. As the probe approaches the surface, the amplitude of cantilever oscillation or the resonant frequency of the cantilever beam changes due to interactions with the surface. A feedback loop adjusts the height of the scanner to keep the cantilever vibrational amplitude or the cantilever vibrational frequency at a constant value, which also maintains the average tip to sample distance constant, and the height of the scanner at each data point in the scan over the surface is recorded. The low force applied to the sample in the non-contact mode makes it particularly useful for imaging soft samples, for example, DNA-protein complexes. SFM can also be carried in an intermittent contact mode, in which the tip is brought closer to the sample than in a full non-contact mode so that at the bottom of its travel the tip just barely hits the sample.
To extract the tip-sample response from the probe in the non-contact mode or intermittent contact mode, many detection schemes exist, including filtering and rms-to-dc conversion as well as analog to digital conversion and subsequent data processing. To extract the tip signal with the highest possible signal to noise (S/N) ratio, the driving signal can be compared with the probe signal using, for example, logarithmic operational amplifiers, analog dividers or lock-in amplifiers.
The use in SFM of high-frequency probes with resonance frequencies above 500 kHz and into the MHz range offers several potential advantages over conventional probes vibrating at 500 kHz or lower. While a commonly cited advantage is minimized tip and sample damage, high-frequency cantilevers are also better suited to measure extremely small forces. It has been shown that the minimum detectable force of a cantilever can be decreased by decreasing the cantilever's coefficient of viscous damping. See F. Gittes, et al., European Biophysics Journal, Vol. 27, 1998, pp. 75, et seq. Other researchers have shown that the coefficient of viscous damping decreases with decreasing cantilever length and thus with increased resonant frequency. However, the use of a custom designed high-frequency cantilever also requires a custom designed feedback system since standard feedback systems are limited to cantilever vibration frequencies below about 500 kHz.